Power transistor devices, e.g., radio frequency (RF) power transistors, operate at a high junction temperature. It is desirable for power transistor device assemblies to have a high thermal conductivity for proper performance and reliability. Thus, efforts are made during the processing of such power transistor devices to ensure that proper thermal coupling exists between a given device and a heat sink upon which the device is typically mounted, so that sufficient heat removal is attained.
The use of an ultra-thin device, achieved by thinning or otherwise processing the device substrate, is one technique employed to improve the thermal conductivity and thus enhance device performance. However, due to the thinning of the substrate and to stresses associated with device components, bowing of the device often occurs. Bowing presents an issue for handling and attaching the device, e.g., to the heat sink. More specifically, a bowed device is difficult to pick up, place and attach with a uniform and planar bondline.
Typically, reflow mounting is used to solder a power transistor device to a heat sink. Metals are often deposited on the substrate to provide a robust surface for attachment to the heat sink. For example, a titanium (Ti)-containing adhesion layer, deposited directly on the back side of the substrate, is commonly used for a number of solder systems. However, these metals, and Ti in particular, have high inherent stresses that only add to the bowing of the device.
Bowing of the device may cause voids to form between the device and the heat sink, resulting in an unfavorable and non-uniform mating of surfaces, and thus inadequate heat removal characteristics. Accordingly, there exists a need for power transistor devices which exhibit a reduced amount of bowing.